A superhydrophobic surface is a highly water-repellent surface characterized by a resistance to wetting and high water droplet contact angles. FIG. 1 shows a water droplet having a contact angle of θ on an exemplary surface. Surfaces with water droplet contact angles in excess of 90 degrees are generally considered to be hydrophobic. On a smooth surface, a water droplet can theoretically reach a maximum contact angle of 120 degrees. If the apparent water droplet contact angle exceeds 150 degrees, as may occur when a surface includes microscale asperities, the surface may be said to be superhydrophobic. Superhydrophobicity may be referred to as the “lotus leaf effect” given its origins in nature.
The surface of a lotus leaf is covered with countless microscopic protrusions coated with a waxy layer. This waxy layer acts as a multifunctional interface between the leaf and its environment, influencing airflow and light reflection, and imparting, along with the protrusions, very high water repellency to the surface of the leaf. Water falling on the leaf rolls over the surface as small droplets. The hydrophobic topographical microfeatures minimize the area of contact between a water droplet and the leaf surface, thereby keeping the droplet in contact mainly with the surrounding air. As a result, the water on the leaf surface substantially retains the droplet shape it would have in the air. The rough, waxy microstructures present on the lotus leaf result in contact angles as high as 170°, thereby imparting to the surface enhanced superhydrophobic properties.
Surfaces may be roughened, patterned, or otherwise processed to obtain the microscale features deemed advantageous for superhydrophobicity. Superhydrophobic coatings may also be formed on a hydrophilic surface to impart superhydrophobic characteristics to the surface. For example, superhydrophobic coatings can be produced on glass to form windows with self-cleaning capabilities. Challenges remain, however, in producing coatings that are well-bonded to the underlying surface and which exhibit both excellent superhydrophobic properties and optical transparency.